Along with recent advancements in smaller and higher speed electrophotographic devices with higher image quality, there is a strong demand for improving low-temperature fixability of the toner in view of energy saving by reducing the amount of energy consumption in a fixing step.
Usually, a method that reduces the glass transition temperature of a binding resin is used to reduce the fixing temperature of the toner.
However, if the glass transition temperature is reduced too much, the hot offset resistance will be reduced and aggregation of powder (i.e., “blocking”) will easily occur, thus reducing the storage stability of the toner. Therefore, the practical lower limit of the glass transition temperature is 50° C. The glass transition temperature is a design point of the binding resin, and the method that reduces the glass transition temperature cannot provide a toner that can be fixed at even lower temperatures.
Patent Literatures 1 and 2 disclose toner compositions containing a polyester-based toner binder. These toner compositions are excellent in low-temperature fixability and hot offset resistance. Yet, a recent demand to ensure storage stability and maintain the balance between low-temperature fixability and hot offset resistance (fixing temperature range) is further increasing, and the above toner compositions are yet to meet the demand.
In another method, a combination of an amorphous resin and a crystalline resin is used for a binding resin. It is known that such a combination improves the low-temperature fixability and gloss of the toner due to the melt characteristics of the crystalline resin.
Yet, in some cases, a higher crystalline resin content reduces the resin strength, and the crystalline resin becomes amorphous during melt-kneading due to miscibility between the crystalline resin and the binding resin, resulting in a decrease in the glass transition temperature of the toner, thus causing the same problems as mentioned above.
Some methods are suggested as countermeasures to the above problems. For example, Patent Literature 3 discloses a method for recrystallizing the crystalline resin by a heat treatment after a melt-kneading step, and Patent Literatures 4 and 5 each disclose a method in which different monomer components are used.
With the above methods, it is possible to ensure low-temperature fixability and gloss of the toner; however, properties such as hot offset resistance, toner flowability, and heat-resistant storage stability (i.e., stability during storage at high temperatures) are insufficient. These methods are also faced with problems such as a decrease in electrostatic stability and grindability during grinding.
Patent Literatures 6 to 9 each suggest a method in which the core is encapsulated by a shell layer obtained by a melt suspension method or an emulsification aggregation method. Yet, the crystalline resin is miscible with the binding resin as the core, and the crystals cannot be sufficiently re-precipitated in a short time. Thus, it is still not possible to provide sufficient image strength after fixing or sufficient folding resistance.
In addition, Patent Literature 10 discloses a method in which a crystalline resin is added to a styrene-acrylic based amorphous resin, and crystal precipitation is induced by immiscibility between the amorphous resin and the crystalline resin. Yet, since the amorphous resin is a styrene acrylic resin, the resulting toner has sufficient low-temperature fixability.